linux-kernel - Re: [PATCHv4 03/24] mm: avoid PG

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Message-ID: <20150304205614.GA19606@node.dhcp.inet.fi>
Date:	Wed, 4 Mar 2015 22:56:14 +0200
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Christoph Lameter <cl@...ux.com>
Cc:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Hugh Dickins <hughd@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Rik van Riel <riel@...hat.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Steve Capper <steve.capper@...aro.org>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	Jerome Marchand <jmarchan@...hat.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCHv4 03/24] mm: avoid PG_locked on tail pages

On Wed, Mar 04, 2015 at 12:48:54PM -0600, Christoph Lameter wrote:
> On Wed, 4 Mar 2015, Kirill A. Shutemov wrote:
> 
> > index c851ff92d5b3..58b98bced299 100644
> > --- a/include/linux/page-flags.h
> > +++ b/include/linux/page-flags.h
> > @@ -207,7 +207,8 @@ static inline int __TestClearPage##uname(struct page *page) { return 0; }
> >
> >  struct page;	/* forward declaration */
> >
> > -TESTPAGEFLAG(Locked, locked)
> > +#define PageLocked(page) test_bit(PG_locked, &compound_head(page)->flags)
> > +
> >  PAGEFLAG(Error, error) TESTCLEARFLAG(Error, error)
> 
> Hmmm... Now one of the pageflag functions operates on the head page unlike
> the other pageflag functions that only operate on the flag indicated.
> 
> Given that pageflags provide a way to implement checks for head / tail
> pages this seems to be a bad idea.

I agree, I need to take more systematic look on page flags vs. compound
pages. I'll try to come up with something before the summit.

-- 
 Kirill A. Shutemov
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